/Engine/Plugins/Runtime/RigVM/Source/RigVMDeveloper/Public/RigVMCompiler/RigVMAST.h
An abstract syntax tree call extern expression represents the invocation of an extern function. In C++ the call extern is an invocation:FMath::Clamp(1.4, 0.0, 1.0); The call extern expression references a node (through parent class) from the model providing all of the relevant information for the invocation.
class FRigVMCallExternExprAST : public [FRigVMNodeExprAST](API\Plugins\RigVMDeveloper\RigVMCompiler\FRigVMNodeExprAST)